1. Field of the Invention
The field of the present invention relates to sound reproduction and, more specifically, to speaker configurations and enclosures, and related sound processing.
2. Background
Sound reproduction systems incorporating speakers are commonplace in homes, theaters, automobiles, places of entertainment, and elsewhere. The number, size, quality, characteristics, and arrangement of speakers affect sound quality in virtually any listening environment. However, many environments have constraints which limit the number, size, or type of speakers which can be used, and their arrangement. These constraints may be technical, mechanical, or aesthetic in nature.
For example, with respect to consumer products such as computers and televisions, there may be limited space to physically attach or integrate speakers. A common practice is to provide a set of external speakers separate from the enclosure of the computer, television, or other product, allowing the user the ability to place the speakers widely apart and thus achieve a true stereo effect. However, loose speakers take up space on a desk or table, and require unsightly or inconvenient electrical connections to the computer, television, or other product. Moreover, use of such additional external speakers generally requires the consumer to purchase them separately from the main product itself, thus increasing cost. In addition, space restrictions on a desk or table may limit the possible locations of speakers, and/or their number, size and orientation, and thus adversely affect sound quality including the desired stereo effect.
For consumer items such as laptop computers, the option of utilizing external speakers to improve sound quality may not be possible.
Confined listening areas also create constraints which can impact sound quality, and can often unsuitable for optimal sound reproduction. For example, the listening space of an automobile creates particular challenges and problems for quality sound reproduction. These problems partially result from the unique sound environment of the automobile when compared with a good listening room. Among the disadvantages are:                Much smaller internal volume resulting in a reduced reverberation time and lower modal density at low frequencies resulting in a lack of ambience and an uneven bass response.        The proximity of highly reflective surfaces (such as the windows) to highly absorptive areas such as the upholstery or the occupants clothing leads to a great variability with frequency and head position of the direct to indirect sound arriving at the listener. Consequently even small changes in head or seating position can cause significant and undesirable changes in the timbral quality of the music.        The listening positions are necessarily restricted to the seating positions provided (usually 4 or 5) and all of these are very asymmetrically placed with respect to the speaker positions. Space is always at a premium within a car interior and as a result the speakers are often placed in physically convenient positions, that are nevertheless very poor from an acoustic point of view, such as the foot wells and the bottom of the front and rear side doors. As a result the listener's head is always much closer to either the left or right speaker leading directly large inter-channel time differences and different sound levels due to the 1/r law.        Additionally, the angles between the axes from the listeners ears to the axes of symmetry of the left and right speakers is quite different for each occupant. The perceived spectral balance is different for each channel due to the directional characteristics of the drive units. Masking of one or more speakers by the occupants clothes or legs can often result in the attenuation of the mid- and high-frequencies by as much as 10 dB.        
The conditions noted above tend to adversely impact the ability to produce high quality stereo reproduction, which ideally has the following attributes:                A believable and stable image or soundstage resulting from the listener being nearly equidistant from the speakers reproducing the left and right channels and a sufficiently high ratio of direct-to-indirect sound at the listener's ears.        A smooth timbral balance at all the listening positions.        A sense of ambience resulting from a uniform soundfield.        
Some features are provided in automobile audio systems which can partially mitigate the aforementioned problems. For example, an occupant can manually adjust the sound balance to increase the proportional volume to the left or right speakers. Some automobile audio systems have a “driver mode” button which makes the sound optimal for the driver. However, because different listening axes exist for left and right occupants, an adjustment to the balance that satisfies the occupant (e.g., driver) on one side of the automobile will usually make the sound worse for the occupant seated on the other side of the automobile. Moreover, balance adjustment requires manual adjustment by one of the occupants, and it is generally desirable in an automobile to minimize user intervention.
Another modification made to some automobile audio systems is to provide a center speaker, which reduces the image instability that occurs when the listener is closer to either the left or right speaker when both are reproducing the same mono signal, with the intention of producing a central sound image. Yet another possible approach is adding more speakers in a greater variety of positions (e.g., at the seat tops). While such techniques can sometimes provide a more pleasing effect, they cannot provide stable imaging as the problems associated with asymmetry described above still remain. The considerable additional cost of such design approaches is usually undesirable in markets such as the highly cost sensitive and competitive automotive industry. Moreover, as previously noted, space is usually at a premium in the automobile interior, and optimal speaker positions are limited.
The aforementioned problems are not limited to sound systems designed for automobiles, but may exist in other confined spaces as well. Even in larger spaces, it may be difficult to achieve ideal sound reproduction due to constraints on where speakers may be located, or other considerations. Freestanding speakers can take up valuable room space, while speakers embedded in walls and ceilings require a large cross-sectional areas and may be aesthetically displeasing. More generally, in many environments it is desirable to minimize the visual impact of speakers in a sound reproduction system. One technique, for example, is to color or otherwise decorate the protective speaker faceplate to match the surrounding wall or object in which the speaker in placed, or to hide speakers behind an artificial painting. These types of solutions may not be satisfactory for all consumers, and may limit the possibilities for optimal speaker placement as well.
It would therefore be advantageous to provide an improved sound reproduction and/or speaker system which overcomes the foregoing problems, and/or provides other benefits and advantages.